Cardiac therapy triggered by capture verification

ABSTRACT

The present invention provides methods and systems for tachyarrhythmia therapy involving pacing the heart to prior to the application of a cardioversion/defibrillation shock. One or more pace pulses are delivered to the arrhythmic chamber or chambers. The pace pulses may be delivered to the heart at an adaptable rate selected to organize the electrical activity of the heart. If the pace pulses produce capture, cardioversion/defibrillation stimulation is delivered.

RELATED PATENT DOCUMENTS

This application is a division of U.S. patent application Ser. No.10/377,257 filed on Feb. 28, 2003 which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to providing cardiac therapyand, more particularly, to providing tachyarrhythmia therapy.

BACKGROUND OF THE INVENTION

When functioning normally, the heart produces rhythmic contractions andis capable of pumping blood throughout the body. However, due to diseaseor injury, the heart rhythm may become irregular resulting in diminishedblood circulation. Arrhythmia is a general term used to describe heartrhythm irregularities arising from a variety of physical conditions anddisease processes.

An abnormally fast heart rate is designated tachyarrhythmia.Tachyarrhythmias may originate in either the atria or the ventricles.Tachycardia is a term generally used to describe cardiac rhythms thatare rapid, but relatively organized. Conversely, fibrillation ischaracterized by rapid, chaotic, and disorganized heart rhythms.Tachycardia and fibrillation may affect either the atria or theventricles.

Pacemakers have been used as an effective treatment for patients withserious arrhythmias such as bradycardia, a condition characterized by anabnormally slow heart rate. Pacemakers typically include circuitry tosense electrical signals from the heart and a pulse generator fordelivering a series of low energy electrical stimulation pulses to theheart. Leads extending into the patient's heart are connected toelectrodes that contact the myocardium for sensing the heart'selectrical signals and for delivering stimulation pulses to the heart.The pace pulses may be intermittent or continuous, depending on thepatient's metabolic demand. The pace-pulses are timed to assist theheart in producing a contractile rhythm that maintains cardiac pumpingefficiency.

When a pace pulse produces a contractile response in heart tissue, thecontractile response is typically referred to as capture, and theelectrical cardiac signal corresponding to capture is denoted the evokedresponse. A pace pulse must exceed a minimum energy value, or capturethreshold, to produce a contraction. Detection of the evoked responsemay be used to verify that the pace pulse has produced capture of theheart tissue.

Cardiac rhythm management systems may include both a pacemaker and acardioverter/defibrillator. An implantable cardiaccardioverter/defibrillator (ICD) typically monitors cardiac activity anddelivers high energy electrical stimulation to the heart to interrupt atachycardia or fibrillation condition. In general, the ICD continuouslymonitors cardiac activity by analyzing electrical signals, known aselectrograms (EGMs), detected by endocardial sensing electrodes. ICDsare generally capable of diagnosing the various types oftachyarrhythmias discussed above, and then delivering an appropriateelectrical stimulation therapy to the patient's heart to terminate thediagnosed arrhythmia.

In general, atrial tachyarrhythmias are chronic conditions that arenon-life threatening, because the atria only aid in the movement ofblood into the ventricles, where the major pumping action of the heartoccurs. Conversely, ventricular tachyarrhythmia is a life-threateningevent because the heart's ability to pump blood to the rest of the bodyis seriously impaired if the ventricles become arrhythmic.

In the treatment of chronic cardiac conditions, such as atrialtachycardia or fibrillation, the patient is typically conscious and canfeel the electrical stimulation applied to the heart. Thus, it isdesirable to reduce the energy of the electrical stimulation fortreating arrhythmias, particularly chronic atrial arrhythmias. For bothatrial and ventricular tachyarrhythmias, the relative organizationalstate of the tachyarrhythmia may be related to the amount of energyneeded for successful defibrillation therapy.

SUMMARY OF THE INVENTION

The present invention is directed to method and systems providinganti-tachyarrhythmia therapy. One embodiment of the invention involves amethod for providing cardiac therapy to the heart. Pace pulses aredelivered to an atrium during a tachyarrhythmic episode. Capture of theatrium associated with at least one of the pace pulses is detected.Cardioversion/defibrillation stimulation is delivered to the atriumresponsive to the detection of capture.

In accordance with another embodiment of the invention, a method forproviding therapy to a heart involves delivering pace pulses to theheart during a tachyarrhythmia episode. Capture of heart tissueassociated with at least one of the pace pulses is detected.Cardioversion/defibrillation stimulation is delivered to the heart inresponse to detection of capture.

Yet another embodiment of the invention involves a system for treatingcardiac arrhythmia. The system includes a lead system comprisingelectrodes and extending into the heart. Pulse generator circuitry iscoupled to the lead system and configured to generate stimulation pulsesto electrically stimulate the heart. A detector system coupled to thelead system is configured to sense cardiac signals transmitted throughthe lead electrodes. A control system, coupled to the detector systemand the pulse generator, is configured to control the generation of pacepulses, to detect capture associated with at least one of the pacepulses, and to control the delivery of a cardioversion/defibrillationstimulation to the heart responsive to the detection of capture.

In accordance with another embodiment of the invention, a therapy systemincludes means for delivering pace pulses to an atrium of the heartduring a tachyarrhythmia episode, means for detecting capture of theatrium associated with at least one of the pace pulses, and means fordelivering cardioversion/defibrillation stimulation to the atriumresponsive to the detection of capture.

In accordance with yet another embodiment of the invention, a system forproviding therapy to a heart includes means for delivering pace pulsesto the heart during a tachyarrhythmic episode, means for detectingcapture associated with at least one of the pace pulses, and means fordelivering a cardioversion/defibrillation stimulation in response to theverification of capture.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial view of one embodiment of an implantable medicaldevice with an endocardial lead system extending into atrial andventricular chambers of a heart;

FIG. 2 is a block diagram of an implantable medical device with whichanti-tachyarrhythmia therapy methods may be implemented in accordancewith embodiments of the present invention;

FIG. 3 is a flowchart of a method of providing cardiac therapy inaccordance with embodiments of the present invention;

FIG. 4 is a graph illustrating the effect of pacing on cardiac activityduring atrial fibrillation in accordance with embodiments of theinvention;

FIG. 5 is a flowchart of a method of providing atrial fibrillationtherapy in accordance with embodiments of the present invention; and

FIG. 6 is a flowchart of a method for determining entrainment by pacepulses delivered to the heart during an arrhythmic episode in accordancewith embodiments of the invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings forming a part hereof, and inwhich are shown by way of illustration, various embodiments by which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

Pacing an arrhythmic heart chamber prior to the delivery ofcardioversion/defibrillation shock can decrease the energy required forcardioversion or defibrillation. In accordance with principles of theinvention, pacing can be used to organize the electrical cardiacactivity so that lower cardioversion/defibrillation energy may be usedto terminate the tachyarrhythmia. The present invention describesvarious methods and systems for triggering cardioversion/defibrillationtherapy using capture verification following the application of one ormore pace pulses to the arrhythmic chamber or chambers. In accordancewith various embodiments, pace pulses are delivered to a heart chamberduring a tachyarrhythmia episode. Upon verification of capture of theheart chamber, cardioversion/defibrillation stimulation is applied toterminate the arrhythmia.

The pace pulses may be delivered to the heart chamber in a timedrelationship with the tachyarrhythmic activity. For example, the pacingpulses may be delivered at an adaptable frequency selected to producecapture in the arrhythmic heart chamber and/or to organize theelectrical activity of the heart chamber. Prior to deliveringcardioversion/defibrillation stimulation, the system determines if thepace pulses produce capture in the tachyarrhythmic chamber.

A cardiac cycle length may be measured and used to determine aneffective pacing rate to effect capture and organize the arrhythmiccondition. In one embodiment, a pacing interval is selected as apercentage of the cardiac cycle length. The pacing rate may be furthermodified based on current capture status. If the pace pulses deliveredat the selected pacing interval do not consistently capture thearrhythmic heart chamber, the pacing interval may be modified to promotecapture. When capture occurs consistently, the heart chamber is said tobe entrained by the pace pulses. If the pace pulses entrain theelectrical activity of the heart chamber, the pacing interval may beadapted to improve organization of the tachyarrhythmic activity. Forexample, if entrainment is detected, the pacing interval may belengthened to slow the tachyarrhythmic activity.

The embodiments of the present invention illustrated herein aregenerally described as being implemented in an implantable cardiacdefibrillator (ICD) that may operate in numerous pacing modes known inthe art. Various types of single and multiple chamber implantablecardiac defibrillators are known in the art and may implement a captureverification methodology of the present invention. The systems andmethods of the present invention may also be implemented in a variety ofimplantable or external therapeutic cardiac rhythm management systems,including single and multi chamber pacemakers, resynchronizers, andcardioverter/defibrillator systems, for example. Furthermore, thesystems and methods of the invention may be implemented in diagnosticcardiac devices, such as cardiac monitors and the like, in combinationwith devices that stimulate the heart.

Although the present system is described in conjunction with animplantable cardiac defibrillator having a microprocessor-basedarchitecture, it will be understood that the implantable cardiacdefibrillator (or other device) may be implemented in any logic-basedintegrated circuit architecture, if desired.

Referring now to FIG. 1 of the drawings, there is shown one embodimentof a cardiac rhythm management system that includes an implantablecardiac defibrillator 100 electrically and physically coupled to anintracardiac lead system 102. The intracardiac lead system 102 isimplanted in a human body with portions of the intracardiac lead system102 inserted into a heart 101. The intracardiac lead system 102 is usedto detect and analyze electrical cardiac signals produced by the heart101 and to provide electrical energy to the heart 101 under certainpredetermined conditions to treat cardiac arrhythmias, including, forexample, ventricular fibrillation of the heart 101.

The intracardiac lead system 102 includes one or more pacing electrodesand one or more defibrillation electrodes. In the particular embodimentshown in FIG. 1, the intracardiac lead system 102 includes a ventricularlead system 104 and an atrial lead system 106. The ventricular leadsystem 104 may include an SVC-coil 116, an RV-coil 114, and an RV-tipelectrode 112. The RV-coil 114, which may alternatively be an RV-ringelectrode, is spaced apart from the RV-tip electrode 112, which is apacing electrode. In one embodiment, the ventricular lead system 104 isconfigured as an integrated bipolar pace/shock lead. In anotherexemplary configuration, one or more additional electrodes, e.g., a ringelectrode, may be included in the ventricular lead system 104. Theadditional ring electrode and the RV-tip electrode 112 may be used forbipolar sensing of cardiac signals. The atrial lead system 106 includesan A-tip electrode 152 and an A-ring electrode 154. In one embodiment,the atrial lead system 106 is configured as an atrial J lead.

In this configuration, the intracardiac lead system 102 is positionedwithin the heart 101, with portions of the atrial lead system 106extending into the right atrium 120 and portions of the ventricular leadsystem 104 extending into the right atrium 120 and right ventricle 118.In particular, the A-tip electrode 152 and A-ring electrode 154 arepositioned at appropriate locations within the right atrium 120. TheRV-tip electrode 112 and RV-coil 114 electrodes are positioned atappropriate locations within the right ventricle 118. The SVC-coil 116is positioned at an appropriate location within the right atrium chamber120 of the heart 101 or a major vein leading to the right atrium chamber120 of the heart 101. The RV-coil 114 and SVC-coil 116 depicted in FIG.1 are defibrillation electrodes.

Additional pacing and defibrillation electrodes may also be included inthe intracardiac lead system 102 to allow for various bipolar sensing,pacing, and defibrillation capabilities. For example, the intracardiaclead system 102 may include endocardial sensing, pacing, and/orcardioversion/defibrillation leads (not shown) that are advanced intothe coronary sinus and coronary veins to locate the distal electrode(s)adjacent to the left ventricle or the left atrium. Other intracardiaclead and electrode arrangements and configurations known in the art arealso possible and considered to be within the scope of the presentsystem.

The ventricular and atrial lead systems 104, 106 include conductors forcommunicating sense signals and stimulation pulses between the cardiacdefibrillator 100 and the electrodes and coils of the lead systems 104,106. As is shown in FIG. 1, ventricular lead system 104 includes aconductor 108 for transmitting sense signals and stimulation pulsesbetween the RV-tip electrode 112 and an RV-tip terminal 202 of thecardiac defibrillator 100. A conductor 110 of the ventricular leadsystem 104 transmits sense signals and/or stimulation pulses between theRV-coil electrode 114 and an RV-coil terminal 204 of the cardiacdefibrillator 100. The ventricular lead system 104 also includes aconductor 122 for transmitting sense signals and/or stimulation pulsesbetween and SVC coil terminal 206 of the cardiac defibrillator 100 andthe SVC coil 116. The atrial lead system 106 includes conductors 132,134 for transmitting sense and stimulation signals between terminals212, 210 of the cardiac defibrillator 100 and A-tip and A-ringelectrodes 152 and 154, respectively.

Referring now to FIG. 2, there is shown an embodiment of a cardiacdefibrillator 200 suitable for implementing a capture verificationmethodology of the present invention. FIG. 2 shows a cardiacdefibrillator divided into functional blocks. It is understood by thoseskilled in the art that there exist many possible configurations inwhich these functional blocks can be arranged. The example depicted inFIG. 2 is one possible functional arrangement. The cardiac defibrillator200 includes circuitry for receiving cardiac signals from a heart (notshown in FIG. 2) and delivering electrical stimulation energy to theheart.

In one embodiment, the cardiac defibrillator circuitry 203 of thecardiac defibrillator 200 is encased and hermetically sealed in ahousing 201 suitable for implanting in a human body as is known in theart. Power to the cardiac defibrillator 200 is supplied by anelectrochemical battery 233 that is housed within the cardiacdefibrillator 200. The cardiac defibrillator 200 includes a connectorblock attached to the housing 201 of the cardiac defibrillator 200.Terminals of the connector block allow for the physical and electricalattachment of the intracardiac lead system conductors to the cardiacdefibrillator 200 and the encased cardiac defibrillator circuitry 203,as previously discussed.

The cardiac defibrillator circuitry 203 of the cardiac defibrillator 200may be a programmable microprocessor-based system, including a controlsystem 220 and a memory circuit 236. The memory circuit 236 storesparameters for various pacing, defibrillation, and sensing modes, andstores data indicative of cardiac signals received by other componentsof the cardiac defibrillator circuitry 203. The control system 220 andmemory circuit 236 cooperate with other components of the cardiacdefibrillator circuitry 203 to perform operations involving therapydelivery based on capture verification according to the principles ofthe present invention, in addition to other sensing, pacing anddefibrillation functions. The control system 220 may encompassadditional elements, including a pacemaker 222, and an arrhythmiadetector 240, along with other functional components, for controllingthe cardiac defibrillator circuitry 203. A memory 232 is also providedfor storing historical EGM and therapy data. The historical data may beused for various purposes to control the operations of the cardiacdefibrillator 200 and may also be transmitted to an external programmerunit 234 as needed or desired.

Telemetry circuitry 231 is additionally coupled to the cardiacdefibrillator circuitry 203 to allow the cardiac defibrillator 200 tocommunicate with an external programmer unit 234. In one embodiment, thetelemetry circuitry 231 and the programmer unit 234 use a wire loopantenna and a radio frequency telemetric link, as is known in the art,to receive and transmit signals and data between the programmer unit 234telemetry circuitry 231. In this manner, programming commands may betransferred to the control system 220 of the cardiac defibrillator 200from the programmer unit 234 during and after implant. In addition,stored cardiac data pertaining to the functioning of the heart, alongwith other data, may be transferred to the programmer unit 234 from thecardiac defibrillator 200, for example.

Cardiac signals sensed through use of the RV-tip electrode 112 may besensed as near-field signals as are known in the art. More particularly,a near-field signal may be detected as a voltage developed between theRV-tip electrode 112 and the RV-coil 114. Cardiac signals sensed throughuse of one or both of the defibrillation coils or electrodes 114, 116are far-field cardiac signals, as are known in the art. Moreparticularly, for example, a far-field signal is detected as a voltagedeveloped between the RV-coil 114 and the SVC-coil 116.

A far-field cardiac signal may be detected as a voltage developedbetween the RV-coil 114 and the can electrode 209. Alternatively, thecan electrode 209 and the SVC-coil electrode 116 may be shorted and thecardiac signal sensed as the voltage developed between the RV-coil 114and the can electrode 209/SVC-coil 116 combination. Cardiac signalsdeveloped using appropriate combinations of the RV-coil, SVC-coil, andcan electrodes 114, 116 and 209 are sensed and amplified by an EGMamplifier 229. The output of the EGM amplifier 229 is coupled to thecontrol system 220.

In the embodiment of the cardiac defibrillator 200 depicted in FIG. 2,RV-tip and RV-coil electrodes 112, 114 are shown coupled to a V-senseamplifier 226 and thus to an R-wave detector 223. Cardiac signalsreceived by the V-sense amplifier 226 are communicated to the R-wavedetector 223, which serves to sense and amplify the cardiac signals,e.g. R-waves. The sensed R-waves may then be communicated to the controlsystem 220.

A-tip and A-ring electrodes 152, 154 are shown coupled to an A-senseamplifier 225. Atrial sense signals received by the A-sense amplifier225 are communicated to an A-wave detector 221, which serves to senseand amplify the A-wave signals. The atrial signals may be communicatedfrom the A-wave detector 221 to the control system 220.

The pacemaker 222 communicates pacing signals to the RV-tip and A-tipelectrodes 112 and 152 according to a preestablished pacing regimenunder appropriate conditions. Blanking circuitry (not shown) is employedin a known manner when a ventricular or atrial pacing pulse isdelivered, such that the ventricular or atrial channels are properlyblanked at the appropriate time and for the appropriate duration.

A switching matrix 228, according to one embodiment, may be coupled tothe A ring 154, RV tip 112, RV coil 114 and SVC coil 116 electrodes. Theswitching matrix 228 can be arranged to provide connections to variousconfigurations of pacing and defibrillation electrodes. The outputs ofthe switching matrix 228 are coupled to an evoked response (ER)amplifier 227 which serves to sense and amplify signals detected betweenthe selected combinations of electrodes. The detected signals arecoupled through the ER amplifier to a capture detector 224. The capturedetector 224 includes circuitry configured to detect an evoked responseproduced by a pace pulse.

The cardiac defibrillator 200 depicted in FIG. 2 is well-suited forimplementing a tachyarrhythmia therapy methodology according to theprinciples of the present invention. In the embodiment illustrated inFIG. 2, the arrhythmia detector 240 receives signals representingcardiac activity and detects arrhythmias of the heart. The capturedetection processes of the present invention are primarily carried outby the capture detector 224 in cooperation with other components of thecontrol system 220.

Among other functions, the control system 220 controls the delivery oftherapy to terminate a tachyarrhythmia of the heart in accordance withthe principles of the invention. The therapy may include, for example,delivering low energy pace pulses to the heart using the pacemakercontrol circuitry 222 and the pace pulse generator 242 located in thepulse generator 230. Furthermore, the therapy may include deliveringhigh energy cardioversion/defibrillation pulses to one or more heartchambers using the cardioverter/defibrillator pulse generator 241triggered by detection of capture.

FIG. 3 is a flowchart illustrating a method of providing tachyarrhythmiatherapy in accordance with embodiments of the invention. Followingidentification of a tachyarrhythmic episode, one or more pace pulses aredelivered 310 to the arrhythmic chamber during the tachyarrhythmicepisode. The energy of the pace pulses may be selected to producecapture of the arrhythmic tissue. The pace pulses are delivered at anenergy above the sinus rhythm capture threshold. For example, the energyof the pace pulses may be delivered at twice the sinus rhythm capturethreshold, or a maximum pacing energy.

The pace pulses may be delivered at a pacing rate selected to producecapture of the atrium. In one example, the pace pulses are initiallydelivered at a predetermined percentage of the cardiac cycle length,such as 80-95% of the cardiac cycle length. The pacing rate may beadjusted to increase the cardiac cycle length prior to application ofthe cardioversion/defibrillation stimulation.

Following delivery of the pace pulses, capture of the arrythmic tissueis verified 320. The energy and frequency of the pace pulses areselected to capture all or a portion of the fibrillating tissue and thus“entrain” the cardiac activity. The evoked responses produced by thepace pulses represent more organized cardiac activity when compared tothe chaotic activity of fibrillation. After entrainment, the pacing ratemay be further adjusted to enhance organization while maintainingcapture. If entrainment is not detected within a selected time interval,the pacing therapy may be terminated.

Capture verification may be implemented in the capture detector 224,shown in FIG. 2, using various techniques. The capture detector maydetect various features of a cardiac waveform consistent with an evokedresponse morphology to determine if capture occurs at a pacing site. Anexemplary set of features that may be used to determine capture includean amplitude of a cardiac signal, slope of the cardiac signal, timing oflocal maxima or minima of the cardiac signal, the rise time and/or falltimes of the cardiac signal, or a curvature of the cardiac signal. Otherfeatures of the cardiac signal may also be used to determine capture.

In one embodiment, the capture detector determines capture has occurredby comparing an amplitude of a sensed cardiac signal within a specifiedtime window following the stimulation pulse to an amplitude associatedwith an evoked response. If the sensed cardiac signal achieves theamplitude associated with the evoked response, indicating capture of thepaced chamber or chambers, the capture detector determines that capturehas occurred. In other embodiments, one or more time intervals betweencardiac signal features may also be used to determine capture. Capturemay also be determined by comparing a cardiac waveform produced by astimulus pulse to an evoked response template waveform.

Following capture detection, a cardioversion/defibrillation stimulus isdelivered 330 to the heart. In accordance with embodiments of theinvention, detection of capture by one or more pace pulses triggers thedelivery of the cardioversion/defibrillation stimulation. Entrainmentmay be verified if a series of pace pulses result in capture of theheart tissue. Triggering the cardioversion/defibrillation stimulationupon verification of successful entrainment may be used to ensureorganization of cardiac activity prior to delivery of thecardioversion/defibrillation shock.

If the pace pulses applied capture a sufficient area of arrhythmic hearttissue, cardiac activity may be more organized, and a lower energy shockmay be used to terminate the arrhythmia. Using lower energy stimulationto terminate the arrhythmia may be more comfortable for the patient andextend the battery life of the ICD.

The delivery of atrial therapy may be enhanced by synchronization of thecardioversion/defibrillation stimulation with a QRS complex.Synchronization of the cardioversion/defibrillation stimulation to a QRScomplex decreases the likelihood of delivering the stimulation pulseduring a period of ventricular vulnerability, a situation that mayinduce ventricular tachyarrhythmia.

As previously discussed, the pacing rate may be adjusted to increase thecardiac cycle length. According to this method, the pacing rate isestablished at a selected percentage of the cardiac cycle. Upondetection of capture, the pacing rate may be decreased, thus increasingthe cardiac cycle length while capture is maintained.

FIG. 4 illustrates therapy delivery for atrial fibrillation inaccordance with an embodiment of the invention. In this example, atrial401 and ventricular 402 activity is illustrated in charts A and V,respectively. The ventricular activity includes a number of QRScomplexes 450. Cardiac activity during atrial fibrillation 410 ischaracterized by rapid and disorganized depolarizations. Atrialfibrillation is detected 420 and pacing entrainment is attempted. Aseries of pacing pulses 430 are delivered to the atrium. In thisexample, the pacing activity in the atrium does not influence the innateventricular activity.

After the pace pulses consistently capture the atrium 440, cardiacactivity becomes more organized, but remains more rapid than a normalrate. The increased organization of the pace pulses decreases the energyrequired to terminate the atrial fibrillation. After entrainment isdetected 440, delivery of the cardioversion/defibrillation stimulation470 to the atrium is synchronized with a ventricular QRS complex 450.

Cardioversion therapy to terminate atrial fibrillation in accordancewith a embodiments of the invention is illustrated in FIG. 5. Althoughthe example provided in FIG. 5 relates to atrial fibrillation therapy,the principles of the invention may be applied to alleviatetachyarrhythmia in any heart chamber or multiple heart chambers.Accordingly, following identification of a tachyarrhythmic episodeoccurring in one or more heart chambers, pace pulses are delivered tothe arrhythmic chamber or chambers in an effort to organize theelectrical activity of the heart. Cardioversion/defibrillationstimulation is delivered after verification that the cardiac activity ofthe chamber has become more organized, e.g., entrained by a series ofpace pulses.

Turning now to FIG. 5, following detection of atrial fibrillation 510,the atrial cycle length is determined 515. One or more pace pulses aredelivered 520 to the atrium during a predetermined time interval. Thepace pulses may be delivered at a rate selected to produce and/ormaintain capture of the atrium. The pacing interval may be adjusted toimprove the organization and/or decrease the rate of atrial contractionsbefore application of cardioversion therapy.

The system determines if the pace pulses consistently produce capture525 in the atrium. When the pace pulses consistently produce capture,the atrium is considered to be entrained. In one example, if about threeto five consecutive atrial pace pulses produce capture, the cardiacactivity may be considered to be entrained by the pace pulses.Entrainment of the cardiac tissue by the pace pulses may increase theeffectiveness of cardioversion therapy and/or lower the energyrequirements of the therapy. If entrainment is detected, a cardioversionstimulation is delivered 540 to the heart synchronized to a QRS complex.

If entrainment does not occur 530 within the predetermined time period,pacing is terminated 535. The system determines if the heart remains inatrial fibrillation 545. If the heart remains in fibrillation,additional pacing followed by cardioversion stimulation may be deliveredusing the triggering method described at blocks 510-540. However, if thecardioversion attempts reach a maximum allowable number 550, forexample, about five attempts, then the therapy is terminated 555.

A method for determining entrainment is illustrated in the flow chart ofFIG. 6. Following the detection of a tachyarrhythmia, a counter is setto one and an initial pace pulse is delivered to the arrhythmic chamber.The cardiac waveform responsive to the initial pace pulse is detected610 and an initial feature set is formed 615 from the detected cardiacwaveform. The initial feature set may include such features as theamplitude, inflection points, slope, timing, and local extrema of thecardiac waveform.

An additional pace pulse is delivered and the cardiac waveformresponsive to the additional pace pulse is detected 620. A feature setis extracted 625 from the cardiac waveform. If the feature set is of thecardiac waveform is consistent 630 with the initial feature set, thenthe counter is incremented 640 and the initial feature set is replaced645 by the feature set acquired at block 625. If the feature set is notconsistent with 630 the initial feature set, then the counter is resetto one 635 and the initial feature set is replaced 645 by the featureset acquired at block 625.

If the counter reaches 650 a maximum value during the predeterminedtherapy interval, then the cardiac activity is entrained 655 by the pacepulses. The maximum counter value may be in the range of about three tofive, for example. If the counter has not reached the maximum value andthe process has not timed out 660, then additional pace pulses aredelivered and the responsive cardiac waveforms are detected and assessedaccording to the processes illustrated at blocks 620-655. However, ifthe process exceeds 660 the predetermined therapy interval, then pacingis discontinued 665.

The methods, devices, and systems of the invention described herein areparticularly well-suited to therapies applied to terminate atrialarrhythmias, such as atrial fibrillation. However, the principles of theinvention are also applicable to enhance anti-tachyarrhythmia therapydirected to any heart chamber. Further, the therapy is not limited to asingle chamber, but may be applied to one or more chambers of the heart.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

1. A system for delivering therapy to a heart, comprising: a leadsystem, the lead system comprising electrodes and extending into theheart; pulse generator circuitry coupled to the lead system, the pulsegenerator circuitry configured to generate pulses to electricallystimulate the heart; a detector system, the detector system coupled tothe lead system and configured to sense cardiac signals transmittedthrough the lead electrodes and to detect atrial arrhythmia; a controlsystem coupled to the detector system and the pulse generator, thecontrol system configured to, following detection of a tachyarrhythmicepisode, control the generation of pace pulses during thetachyarrhythmic episode, form a morphological feature set from cardiacsignals sensed following one or more of the pace pulses; detectentrainment of cardiac activity if features of a predetermined number ofthe cardiac signals sensed following the pace pulses are consistent withthe morphological feature set, and trigger delivery of acardioversion/defibrillation stimulation in response to detection ofentrainment.
 2. The system of claim 1, wherein the pace pulses and thecardioversion/defibrillation stimulation are delivered to an atrium. 3.The system of claim 1, wherein the pace pulses and thecardioversion/defibrillation stimulation are delivered to a ventricle.4. The system of claim 1, wherein the pace pulses are delivered to oneor more heart chambers.
 5. The system of claim 1, wherein thecardioversion/defibrillation stimulation is delivered to one or moreheart chambers.
 6. The system of claim 1, wherein the pace pulses aredelivered at an adaptable rate selected to organize electrical activityof the heart.
 7. The system of claim 1, wherein the control system isconfigured to determine a cardiac cycle length and set a pacing intervalto a predetermined percentage of the cardiac cycle length.
 8. The systemof claim 7, wherein the predetermined percentage is about 80-95%.
 9. Thesystem of claim 1, wherein the pace pulses are delivered to the heart ata selectable energy level.
 10. The system of claim 1, wherein the pacepulses are delivered to the heart at an energy level about twice a sinusrhythm capture threshold.
 11. The system of claim 1, wherein the pacepulses are delivered to the heart at a maximum pacing energy level. 12.The system of claim 1, wherein the stimulation is delivered at aselectable energy level.
 13. The system of claim 1, wherein the controlsystem is configured to deliver the cardioversion/defibrillationstimulation on a next ventricular beat following detection ofentrainment of cardiac activity by the pace pulses.
 14. The system ofclaim 1, wherein the control system is configured to detect entrainmentif a predetermined number of consecutive pace pulses produce capture.15. The system of claim 1, wherein the control system is configured todiscontinue pacing if entrainment of cardiac activity is not detectedwithin a predetermined time interval
 16. The system of claim 1, whereinthe predetermined number is about three to five.
 17. The system of claim1, wherein the control system is configured to detect synchronizationbetween the pace pulses and a QRS complex before delivering thecardioversion/defibrillation stimulation.
 18. A system for deliveringtherapy to a heart, comprising: a lead system, the lead systemcomprising electrodes and extending into the heart; pulse generatorcircuitry coupled to the lead system, the pulse generator circuitryconfigured to generate pulses to electrically stimulate the heart; adetector system, the detector system coupled to the lead system andconfigured to sense cardiac signals transmitted through the leadelectrodes; and a control system coupled to the detector system and thepulse generator, the control system configured to control the generationof pace pulses during a tachyarrhythmic episode, and to determine if thepacing pulses produce entrainment of cardiac activity of thetachyarrhythmic episode based on a morphological feature set formedafter detection of the tachyarrhythmic episode.
 19. The system of claim18, wherein the pace pulses are delivered at an adaptable rate selectedto organize the cardiac activity.
 20. The system of claim 18, whereinthe control system is configured to detect synchronization between thepace pulses and a QRS complex before delivering thecardioversion/defibrillation stimulation.